Although you are in a cold winter weather zone and most likely there’s a vapor retarder beneath the attic’s floor insulation layer I’d go with a no-vapor retarder standard for the attic ventilation
calculations. It’s always best to keep your unconditioned attic as cold as possible.
We’ll be using a formula based on 1 square foot of attic ventilation for every 150 square feet of attic floor space balanced equally between the soffit intake vents and exhaust vents along the ridge section of the roof. For your 1200 square foot attic space:
1200 divided by 150 = 8 square feet of attic ventilation which needs to be evenly distributed between the upper and lower portion of the roof: 8 divided by 2 = 4 sq feet for intake and 4 sq feet for exhaust.
Since all the attic vents have net free air flow specification in inches we need to multiply our results by 144.
4 sq feet x 144 = 576 square inches of attic ventilation for intake and 576 for exhaust.
The next step is to decide which type of vent to use for the soffit; I would recommend continuous soffit vent because it delivers air flow to each and every single rafter bay. You can find specifications on attic ventilation products in your local home improvement store, as an example you can check the AirVent brand here - http://www.airvent.com/homeowner/products/intakeSoffit-specs.shtml
If there’s no labels on the existing roof vents, measure the opening under the vent and compare to a vent with similar dimensions in the store. Remember that the actual roof decking cutout, let’s say 10”x10” will not provide100” of net free area because of the screens and other obstruction that might be part of a vent design. A typical static roof vent (something like on your drawing) provides approximately 50 sq inches of net free area.
I marked your vent locations on the drawing below (just like you’ve mentioned except for the continuous recommendation instead of smaller ones), it should work. I just don’t know how much room there’s in your attic’s rear / almost flat section, hopefully there’s sufficient space between the insulation and the roof decking to provide air flow from the soffit.
Let me know if you have more questions, just click “comment” below